Detecting when a path is antagonistic to most GPU approaches takes time, as does preparing the data however it needs to be prepared on the CPU before being uploaded to the GPU. If you can just fill the whole thing on CPU in that time, you wasted your time even thinking about the GPU.
If you can identify a simple case quickly, it's probably totally a good idea to get the path done on the GPU unless you need to bring the pixels back to the CPU, maybe for writing to disk. The upload and then download can be way slower than just, again, filling on CPU.
If you're filling on GPU and then using on GPU (maybe as a web renderer or something), GPU is probably a big win. Except, this may not actually matter. If there is no need to re-render the path after the first time, it would be dumb to keep re-rendering on the GPU each frame / screen paint. Instead you'd want to put it into a texture. Well.... if you're only rendering once and putting into a texture, this whole conversation is maybe pointless? Then what is simple is probably the best idea. Anyway lots to 2d graphics that goes underappreciated!
The reason for this is that the single 2D application that people most want to speed up is font rendering. And font rendering is also the place where the edge cases are really common.
Rendering everything else (geometric shapes) is trivial by comparison.
Text rendering is really complicated. There is a reason why we have so few text shaping engines.
There's way more to this than drawing circles and rectangles, and these hard cases are why much of path / vector graphics filling still ends up being better on CPU where you can accumulate, sort, etc which takes a lot of the work away. CPU does basically per-Y whereas this is GPU per-pixel so perhaps they're almost equal if the GPU has the square of a CPU power. Obv this isn't quite right but gives you an idea.
Video discussing path filling on CPU (super sampling and trapezoid): https://youtu.be/Did21OYIrGI?t=318 We don't talk about the complex cases but this at least may help explain the simple stuff on CPU for those curious.